FIEID OF THE INVENTION
The invention relates to a method for producing partial electrically conductive contact connections between two conductor structures and, in particular, between two conductor structures, at least one of which is applied to a stabilizing carrier.
The term conductor structure is meant to include any kind of conductive structure, that is to say, in principle, any electrically conductive component, such as, for example, the electrical contacts of SMD and IC modules or the like. In particular, the term embraces conductive structures worked out from metal layers or metal foils and, above all, circuits worked out from conductive foils. The conductive foils may, for example, have a thickness of 12-200 .mu.m or even more.
The conductive structures may be vapor-deposited, adhesively bonded or applied in another known way to a carrier or they may be integrated into the carrier for mechanical reinforcement and insulation. Systems of that type are referred to below as composite conductor systems. Plastics and, in particular, thermoplastics, such as polyimides, polyesters, polyolefins, halogenated polyolefins, if appropriate reinforced polyepoxys and copolymerides or graft polymerides of those polymers, inter alia, are suitable as carriers. The conductor structures may also be surface-treated or surface-refined to improve corrosion protection.
However, problems arise when the conductive connections are made between the conductor structures, especially in the case of multi-layer structures of that type with two or more conductor structures, at least one of which is located on a thermoplastic carrier. Thermal methods present difficulties, since the thermoplastic carrier melts under the effective heat, at least in the region of the direct heat effect. The global introduction of heat (in a furnace, etc.) consequently cannot be employed: Methods with the local input of heat and short-term control are preferred (for example, IR, laser, hot-air methods with a thin nozzle, thermocompression methods, resistance methods, gap soldering rod methods, etc.). The difficulties are particularly serious if a conductor structure applied to a thermoplastic carrier is to be connected conductively to a larger and/or thicker composite conductor system. Those may, for example, be composite conductor systems with carrier foils for SMD modules or for metallic junction frames (leadframes) with at least one integrated IC module. The carrier foils are normally relatively thick and sheet-like in that case. If the thermal energy for making the conductive connection is supplied from the side of the thermoplastic carrier, the energy has to be conducted through the carrier and causes the latter to melt. If the thermal energy is supplied from the other side, that is to say, for example, through a leadframe carrier, a considerable part of the energy being supplied is dissipated through the leadframe which, as a rule, is formed of highly heat-conductive metal. On one hand, that increases the energy consumption necessary for making the conductive connection, and on the other hand the dissipation of heat causes the carrier to heat up in the region around the connecting point. Particularly where a thermoplastic carrier is used, in most cases it melts to such an extent that the multi-layer structure can no longer be used due to deformation or destruction.
Thermal methods can therefore only be employed to a very limited extent or not at all for making the conductive connections. If thermal methods, such as welding, soldering, ultrasonic, thermocompression, thermosonic or laser methods, have been employed at all heretofore for connecting conductor structures to form multi-layer structures which include at least one thermoplastic carrier, it has been only with various restrictions. Either complicated restructuring and new dimensioning of the contact points were carried out or the thermoplastic carriers were replaced by other and more expensive materials. Another possibility was to keep the temperature input low, in order to prevent the thermoplastic material from being damaged. However, that led to a reduction in the strength of the connection being achieved.
Published European Patent Application 0 450 470 A2 describes a generic method for producing electrically conductive connections between two conductor structures, which are respectively connected to a carrier of thermoplastic synthetic material. The composite conductor systems have openings in the region of the contact points of the conductor structures. The composite conductor systems are, for example, heated in a heat press in order to produce electrically conductive connections. That method is related to the known disadvantages.